add option to turn warnings into errors

[ghorn]

In my project whenever I build haddocks I seem to have many warnings of the form

Warning: Some.Module: could not find link destinations for:

Every once in a while I go through and fix them all, but I would prefer if I could have my continuous integration prevent these sort of regressions from going in in the first place. Is it currently possible to turn these warnings into errors? If not, what would it look like to add an option to enable this?

[ghorn]

If I could get some guidance on this I'd be happy to contribute a patch.

[harpocrates]

Thanks for the feature request!

The particular warnings you mention come from here:

haddock/haddock-api/src/Haddock/Interface.hs

Lines 112 to 114 in 1868443

	let (interfaces'', msgs) =
	runWriter $ mapM (renameInterface dflags links warnings) interfaces'
	liftIO $ mapM_ putStrLn msgs

But there are also warnings coming from here:

haddock/haddock-api/src/Haddock/Interface.hs

Line 191 in 1868443

liftIO $ mapM_ putStrLn (nub msgs)

It would be quite nice to have a better reporting mechanism than a direct putStrLn. That mechanism could then be configured with a flag to error out when it encounters warnings (and perhaps even carry along more information with it than just a string). Since all warnings are emitted from inside Ghc, we might even be able to re-use some of GHC's machinery.

[harpocrates]

Here is what would be nice:

• we add a new Opt_WarnMalformedHaddock constructor to GHC's WarningFlag for Haddock related warnings (perhaps we'll be able to re-use a future GHC warning for misplaced Haddocks)

• Haddock can then generate warnings in any GhcMonad monad with

  let msg = makeIntoWarning (Reason Opt_WarnMalformedHaddock) $
            mkWarnMsg dflags loc unqual $
            text "Looks like you might've done something wrong..."
  logWarnings (unitBag msg)

We'd get two things for free: nicely coloured error messages (matching GHC's!) and the ability to set -Werror=malformed-haddock. This is also a good opportunity to add locations to error/warning messages.

[peddie]

@harpocrates, I submitted a new pull request at #969 that approaches this in the way we discussed on IRC.